Worcester 350 combi boiler?

There should be a flexible metal clad hose somewhere just below the boiler going from the cold main pipe to the central heating pipe.
On the cold end will be a tap. Some are like a washing machine tap with a plastic lever, others have a screwdriver slot.
You need to turn the tap on until the pressure gets to about 1 to 1.5 bar. Make sure you turn it back off.
If the boiler has run very low you might have to bleed some of your radiators. Then you'll need to top the pressure up again.
Best to do this with the boiler switched off and cold .

T.h.s
May 2007